Internet Draft                                 L. Yang
        Expiration: March 2003                           Intel Labs
        File: draft-ietf-forces-framework-01.txt       R. Dantu
        Working Group: ForCES                               Netrake Corp.
                                                       T. Anderson
                                                            Intel Labs
                                                       Sept 2002
                          ForCES Architectural Framework
        Status of this Memo
        This document is an Internet-Draft and is in full conformance with
        all provisions of Section 10 of RFC2026.  Internet-Drafts are
        working documents of the Internet Engineering Task Force (IETF), its
        areas, and its working groups.  Note that other groups may also
        distribute working documents as Internet-Drafts.
        Internet-Drafts are draft documents valid for a maximum of six
        months and may be updated, replaced, or obsoleted by other documents
        at any time.  It is inappropriate to use Internet-Drafts as
        reference material or to cite them other than as ``work in
        The list of current Internet-Drafts can be accessed at
        The list of Internet-Draft Shadow Directories can be accessed at
     Conventions used in this document
        The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
        this document are to be interpreted as described in [RFC-2119].
     1. Abstract
        This document defines the architectural framework for ForCES network
        elements (NE), and identifies the associated entities and the
        interaction among them.  This framework is intended to satisfy the
        requirements specified in the ForCES requirements draft [FORCES-
     2. Definitions
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        A set of terminology associated with the ForCES requirements is
        defined in [FORCES-REQ] and we only include the ones that are most
        relevant to this document here.
        Forwarding Element (FE) - A logical entity that implements the
        ForCES protocol.  FEs use the underlying hardware to provide per-
        packet processing and handling as directed by a CE via the ForCES
        Control Element (CE) - A logical entity that implements the ForCES
        protocol and uses it to instruct one or more FEs how to process
        packets.  CEs handle functionality such as the execution of control
        and signaling protocols.
        ForCES Network Element (NE) - An entity composed of one or more CEs
        and one or more FEs.  To entities outside a NE, the NE represents a
        single point of management.  Similarly, a NE usually hides its
        internal organization from external entities.
        Pre-association Phase - The period of time during which a FE Manager
        (see below) and a CE Manager (see below) are determining which FE
        and CE should be part of the same network element. Any partitioning
        of PFEs and PCEs occurs during this phase.
        Post-association Phase - The period of time during which a FE does
        know which CE is to control it and vice versa, including the time
        during which the CE and FE are establishing communication with one
        ForCES Protocol - While there may be multiple protocols used within
        the overall ForCES architecture, the term "ForCES protocol" refers
        only to the ForCES post-association phase protocol (see below).
        ForCES Post-Association Phase Protocol - The protocol used for post-
        association phase communication between CEs and FEs.  This protocol
        does not apply to CE-to-CE communication, FE-to-FE communication, or
        to communication between FE and CE managers.  The ForCES protocol is
        a master-slave protocol in which FEs are slaves and CEs are masters.
        This protocol includes both the management of the communication
        channel (e.g., connection establishment, heartbeats) and the control
        messages themselves. This protocol could be a single protocol or
        could consist of multiple protocols working together.
        FE Manager - A logical entity that operates in the pre-association
        phase and is responsible for determining to which CE(s) a FE should
        communicate.  This process is called CE discovery and may involve
        the FE manager learning the capabilities of available CEs.  A FE
        manager may use anything from a static configuration to a pre-
        association phase protocol (see below) to determine which CE(s) to
        use.  Being a logical entity, a FE manager might be physically
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        combined with any of the other logical entities mentioned in this
        CE Manager - A logical entity that operates in the pre-association
        phase and is responsible for determining to which FE(s) a CE should
        communicate.  This process is called FE discovery and may involve
        the CE manager learning the capabilities of available FEs.  A CE
        manager may use anything from a static configuration to a pre-
        association phase protocol (see below) to determine which FE to use.
        Being a logical entity, a CE manager might be physically combined
        with any of the other logical entities mentioned in this section.
        Pre-association Phase Protocol - A protocol between FE managers and
        CE managers that is used to determine which CEs or FEs to use.  A
        pre-association phase protocol may include a CE and/or FE capability
        discovery mechanism.  Note that this capability discovery process is
        wholly separate from (and does not replace) that used within the
        ForCES protocol.  However, the two capability discovery mechanisms
        may utilize the same FE model.
        FE Model - A model that describes the logical processing functions
        of a FE.
        ForCES Protocol Element - A FE or CE.
     3. Introduction to Forwarding and Control Element Separation (ForCES)
        An IP network element (NE) appears to external entities as a
        monolithic piece of network equipment, e.g., a router, NAT,
        firewall, or load balancer. Internally, however, an IP network
        element (NE) (such as a router or switch) is composed of numerous
        logically separated entities that cooperate to provide a given
        functionality (such as routing or IP switching).  Two types of
        network element components exist: control element (CE) in control
        plane and forwarding element (FE) in forwarding plane (or data
        plane).  Forwarding elements typically are ASIC, network-processor,
        or general-purpose processor-based devices that handle data path
        operations for each packet.  Control elements are typically based on
        general-purpose processors that provide control functionality like
        routing and signaling protocols.
        ForCES aims to define a framework and associated protocol(s) to
        standardize the exchange of information between the control plane
        and the forwarding plane. Having standard mechanisms between the CEs
        and FEs allow these components to be physically separated. This
        physical separation accrues several benefits to the ForCES
        architecture. Separate components would allow component vendors to
        specialize in one component without having to become experts in all
        components. It also allows CEs and FEs from different component
        vendors to interoperate with each other and hence it becomes
        possible for system vendors to integrate together CEs and FEs from
        different component vendors. This translates into a lot more design
        choices and flexibility to the system vendors. Overall, ForCES will
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        enable rapid innovation in both the control and forwarding planes
        while maintaining interoperability. Scalability is also easily
        provided by this architecture in that additional forwarding or
        control capacity can be added to existing network elements without
        the need for forklift upgrades.
        One example of such physical separation is at the blade level.
        Figure 1 shows an example configuration of a router, with two
        control blades and multiple router (forwarding) blades, all
        interconnected into a switch fabric backplane. In such chassis
        configuration, the control blades are the CEs while the router
        blades are FEs, and the switch fabric backplane provides the
        physical interconnect for all the blades. Routers today with this
        kind of configuration use proprietary interface for messaging
        between CEs and FEs. The goal of ForCES is to replace such
        proprietary interface with a standard protocol. With a standard
        protocol like ForCES implemented on all blades, it becomes possible
        for control blades from vendor X and routing blades from vendor Y to
        work seamlessly together in one chassis.
             -------------------------       -------------------------
             |  Control Blade A      |       |  Control Blade B      |
             |       (CE)            |       |          (CE)         |
             -------------------------       -------------------------
                     ^   |                           ^    |
                     |   |                           |    |
                     |   V                           |    V
             |               Switch Fabric Backplane                 |
                    ^  |            ^  |                   ^  |
                    |  |            |  |          à        |  |
                    |  V            |  V                   |  V
                ------------    ------------           ------------
                |Router    |    |Router    |           |Router    |
                |Blade #1  |    |Blade #2  |           |Blade #N  |
                |   (FE)   |    |   (FE)   |           |   (FE)   |
                ------------    ------------           ------------
                    ^  |            ^  |                   ^  |
                    |  |            |  |          à        |  |
                    |  V            |  V                   |  V
             Figure 1. A router configuration example with separate blades.
        Another level of physical separation between the CEs and FEs can be
        at the box level. In such configuration, all the CEs and FEs are
        physically separated boxes, interconnected with some kind of high
        speed LAN connection (like Gigabit Ethernet). These separated CEs
        and FEs are only one hop away from each other within a local area
        network. The CEs and FEs communicate to each other by running
        ForCES, and the collection of these CEs and FEs together become one
        routing unit to the external world. Figure 2 shows such an example.
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        In this example, the same physical interconnect (Ethernet) is shared
        for both CE-to-FE and FE-to-FE communication. However, that does not
        have to be the case. One reason to use different interconnect might
        be that CE-to-FE interconnect does not have to be as fast as the FE-
        to-FE interconnect, so the more expensive fast ports can be saved
        for FE-to-FE.  The separate interconnects may also provide
        reliability and redundancy benefits for the NE.
             -------         -------
             | CE1 |         | CE2 |
             -------         -------
                ^               ^
                |               |
                V               V
         ============================================ Ethernet
             ^       ^       à       ^
             |       |               |
             V       V               V
          -------  -------         --------
          | FE#1|  | FE#2|         | FE#n |
          -------  -------         --------
            ^  |     ^  |            ^  |
            |  |     |  |            |  |
            |  V     |  V            |  V
             Figure 2. A router configuration example with separate boxes.
             |       |       |       |       |       |       |
             |OSPF   |RIP    |BGP    |CAC    |LDP    |à      |
             |       |       |       |       |       |       |
             |               ForCES Interface                |
                                     ^   ^
                             ForCES  |   |data
                             control |   |packets
                             messages|   |(e.g., routing packets)
                                     v   v
             |               ForCES Interface                |
             |       |       |       |       |       |       |
             |LPM Fwd|Meter  |Shaper |NAT    |Classi-|à      |
             |       |       |       |       |fier   |       |
             |               FE resources                    |
                     Figure 3. Examples of CE and FE functions
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        Some examples of control functions that can be implemented in the CE
        include routing protocols like RIP, OSPF and BGP, control and
        signaling protocols like CAC (Call Admission Control), LDP (Label
        Distribution Protocol) for MPLS, etc. Examples of forwarding
        functions in FE include LPM (longest prefix match) forwarder,
        classifiers, traffic shaper, meter, NAT, etc. Figure 3 shows a
        diagram with examples in both CE and FE. Any given NE may contain
        one or many of these CE and FE functions in it. The diagram also
        shows that ForCES protocol is used to transport both the control
        messages for ForCES itself and the data packets that are
        originated/destined from/to the control functions in CE (e.g.,
        routing packets). Section 5.2.4 provides more detail on this.
        A set of requirements for control and forwarding separation is
        identified in [FORCES-REQ]. This document describes a ForCES
        architecture that satisfies the architectural requirements of that
        document and defines a framework for ForCES network elements and
        associated entities to facilitate protocol definition.
     4. Architecture
        This section defines the ForCES architectural framework and the
        associated logical components.  This ForCES framework defines
        components of ForCES NEs including several ancillary components.
        These components may be connected in different kinds of topologies
        for flexible packet processing.
                                | ForCES Network Element              |
         --------------   Fc    | --------------      --------------  |
         | CE Manager |---------+-|     CE 1   |------|    CE 2    |  |
         --------------         | |            |  Fr  |            |  |
               |                | --------------      --------------  |
               | Fl             |         |  |    Fp       /          |
               |                |       Fp|  |----------| /           |
               |                |         |             |/            |
               |                |         |             |             |
               |                |         |     Fp     /|----|        |
               |                |         |  /--------/      |        |
         --------------     Ff  | --------------      --------------  |
         | FE Manager |---------+-|     FE 1   |  Fi  |     FE 2   |  |
         --------------         | |            |------|            |  |
                                | --------------      --------------  |
                                |   |  |  |  |          |  |  |  |    |
                                    |  |  |  |          |  |  |  |
                                    |  |  |  |          |  |  |  |
                                      Fi/f                   Fi/f
                     Figure 4. ForCES Architectural Diagram
        The diagram in Figure 4 shows the logical components of the ForCES
        architecture and their relationships.  There are two kinds of
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        components inside a ForCES network element: control element (CE) and
        forwarding element (FE). The framework allows multiple instances of
        CE and FE inside one NE. Each FE contains one or more physical media
        interfaces for receiving and transmitting packets from/to the
        external world. The aggregation of these FE interfaces becomes the
        NEÆs external interfaces. In addition to the external interfaces,
        there must also exist some kind of interconnect within the NE so
        that the CE and FE can communicate with each other, and one FE can
        forward packets to another FE. The diagram also shows two entities
        outside of the ForCES NE: CE Manager and FE Manager. These two
        entities provide configuration to the corresponding CE or FE in the
        pre-association phase (see Section 5.1). There is no defined role
        for FE Manager and CE Manager in post-association phase, thus these
        logical components are not considered part of the ForCES NE.
        For convenience, the logical interactions between these components
        are labeled by reference points Fp, Fc, Ff, Fr, Fl, and Fi, as shown
        in Figure 4. The FE external interfaces are labeled as Fi/f. More
        detail is provided in Section 4 and 5 for each of these reference
        points. All these reference points are important in understanding
        the ForCES architecture, however, the ForCES protocol is only
        defined over one reference point -- Fp.
        The interface between two ForCES NEs is identical to the interface
        between two conventional routers and these two NEs exchange the
        protocol packets through the external interfaces at Fi/f. ForCES NEs
        connect to existing routers transparently.
     4.1. Control Elements and Fr Reference Point
        It is not necessary to define any protocols across the Fr reference
        point to enable control and forwarding separation for simple
        configurations like single CE and multiple FEs. However, this
        architecture permits multiple CEs to be present in a network
        element. In cases where an implementation uses multiple CEs, it is
        expected the invariant that the CEs and FEs together appear as a
        single NE MUST be maintained.
        Multiple CEs may be used for redundancy, load sharing, distributed
        control, or other purposes.  Redundancy is the case where one or
        more CEs are prepared to take over should an active CE fail.  Load
        sharing is the case where two or more CEs are concurrently active
        and where any request that can be serviced by one of the CEs can
        also be serviced by any of the other CEs.  In both redundancy and
        load sharing, the CEs involved are equivalently capable.  The only
        difference between these two cases is in terms of how many active
        CEs there are.  Distributed control is the case where two or more
        CEs are concurrently active but where certain requests can only be
        serviced by certain CEs.
        When multiple CEs are employed in a ForCES NE, their internal
        organization is considered an implementation issue that is beyond
        the scope of ForCES. CEs are wholly responsible for coordinating
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        amongst themselves via the Fr reference point to provide consistency
        and synchronization. However, ForCES does not define the
        implementation or protocols used between CEs, nor does it define how
        to distribute functionality among CEs. Nevertheless, ForCES will
        support mechanisms for CE redundancy or fail over, and it is
        expected that vendors will provide redundancy or fail over solutions
        within this framework.
     4.2. Forwarding Elements and Fi reference point
        FEs perform per-packet processing and handling as directed by CEs.
        FEs have no initiative of their own.  Instead, FEs are slaves and
        only do as they are told.  FEs may communicate with one or more CEs
        concurrently across reference point Fp.  FEs have no notion of CE
        redundancy, load sharing, or distributed control.  Instead, FEs
        accept commands from any CE authorized to control them, and it is up
        to the CEs to coordinate among themselves to achieve redundancy,
        load sharing or distributed control. The idea is to keep FEs as
        simple and dumb as possible so that FEs can focus its resource on
        the packet processing functions.
                     -------   Fr  -------
                     | CE1 | ------| CE2 |
                     -------       -------
                       |   \      /   |
                       |    \    /    |
                       |     \  /     |
                       |      \/Fp    |
                       |      /\      |
                       |     /  \     |
                       |    /    \    |
                     -------  Fi   -------
                     | FE1 |<----->| FE2 |
                     -------       -------
                         Figure 5. CE redundancy example.
        For example, in Figure 5, FE1 and FE2 can be configured to accept
        commands from both the primary CE (CE1) and the backup CE (CE2). At
        the beginning, CE1 issues commands to FEs while CE2 silently remains
        in sync with CE1 via CE to CE protocol over Fr reference point. When
        CE1 fails, CE2 detects it and starts to take over. Before CE2 starts
        issuing commands to the FEs, it might need to recheck the FEs' state
        and instruct FEs whether or not it is ok to preserve their current
        Distributed control can be achieved in the similar fashion, without
        much intelligence on the part of FEs. For example, FEs can be
        configured to detect RSVP and BGP protocol packets, and forward RSVP
        packets to one CE and BGP packets to another CE. Hence, FEs may need
        to do packet filtering for forwarding packets to specific CEs.
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        This architecture permits multiple FEs to be present in a NE.
        [FORCES-REQ] dictates that the ForCES protocol MUST be able to scale
        to at least hundreds of FEs (see [FORCES-REQ] Section 5, requirement
        #11). Each of these FEs may potentially have a different set of
        packet processing functions, with different media interfaces.  FEs
        are responsible for basic maintenance of layer-2 connectivity with
        other FEs and with external entities.  Many layer-2 media include
        sophisticated control protocols.  The FORCES protocol (over the Fp
        reference point) will be able to carry messages for such protools so
        that, in keeping with the "dumb FE model" the CE can provide
        appropriate intelligence and control over these media.
        When multiple FEs are present, ForCES requires that packets MUST be
        able to arrive at the NE by one FE and leave the NE via a different
        FE (See [FORCES-REQ], Section 5, Requirement #3).  Packets that
        enter the NE via one FE and leave the NE via a different FE are
        transferred between FEs across the Fi reference point.  The Fi
        reference point is a separate protocol from the Fp reference point
        and is not currently defined by the ForCES architecture.
        FEs could be connected in different kinds of topologies and packet
        processing may spread across several FEs in the topology. Hence,
        logical packet flow may be different from physical FE topology.
        Figure 6 provides some topology examples. When it is necessary to
        forward packets between FEs, CE needs to understand the FE topology.
        The FE topology can be queried from FEs by CEs. If the most common
        FE topology is full mesh among FEs, ForCES can assume it as the
        default topology for FEs and hence no query is needed for such
        default cases.
                             |      CE       |
                              ^      ^      ^
                             /       |       \
                            /        v        \
                           /       -----       \
                          /     +->|FE3|<-+     \
                         v      |  -----  |      v
                       -------  |         |  -------
                       | FE1 |<-+         +->| FE2 |
                       -------               -------
                         ^  |                  ^  |
                         |  |                  |  |
                         |  v                  |  v
                     (a) Full mesh among FE1, FE2 and FE3.
                             |   CE    |
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                            ^ ^       ^ ^
                           /  |       |  \
                    /------   |       |   ------\
                    v         v       v          v
                -------   -------   -------   -------
                | FE1 |<->| FE2 |<->| FE3 |<->| FE4 |
                -------   -------   -------   -------
                  ^  |     ^  |       ^  |     ^  |
                  |  |     |  |       |  |     |  |
                  |  v     |  v       |  v     |  v
                     (b) Multiple FEs in a daisy chain
                                ^ |
                                | v
                             |   FE1   |<-----------------------|
                             -----------                        |
                               ^    ^                           |
                              /      \                          |
                       | ^   /        \   ^ |                   V
                       v |  v          v  | v                ----------
                     ---------        ---------              |        |
                     | FE2   |        |  FE3  |<------------>|   CE   |
                     ---------        ---------              |        |
                         ^  ^          ^                     ----------
                         |   \        /                        ^  ^
                         |    \      /                         |  |
                         |    v     v                          |  |
                         |   -----------                       |  |
                         |   |   FE4   |<----------------------|  |
                         |   -----------                          |
                         |      |  ^                              |
                         |      v  |                              |
                         |                                        |
                     (c) Multiple FEs connected by a ring
                     Figure 6. Some examples of FE topology.
     4.3. CE Managers
        CE managers are responsible for determining which FEs a CE should
        control.  It is legitimate for CE managers to be hard-coded with the
        knowledge of with which FEs its CEs should communicate. A CE manager
        may also be physically embedded into a CE and be implemented as a
        simple keypad or other direct configuration mechanism on the CE.
        Finally, CE managers may be physically and logically separate
        entities that configure the CE with FE information via such
        mechanisms as COPS-PR [RFC3084] or SNMP [RFC1157].
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     4.4. FE Managers
        FE managers are responsible for determining to which CE any
        particular FE should initially communicate.  Like CE managers, no
        restrictions are placed on how a FE manager decides to which CEs its
        FEs should communicate, nor are restrictions placed on how FE
        managers are implemented.
     5. Operational Phases
        Both FEs and CEs require some configuration in place before they can
        start information exchange and function as a coherent network
        element. Two operational phases are identified in this framework --
        pre-association and post-association.
     5.1.Pre-association Phase
        Pre-association phase is the period of time during which a FE
        Manager and a CE Manager are determining which FE and CE should be
        part of the same network element. The protocols used during this
        phase may include all or some of the message exchange over Fl, Ff
        and Fc reference points. However, all these may be optional and none
        of this is within the scope of ForCES protocol.
     5.1.1. Fl Reference Point
             FE Manager      FE               CE Manager     CE
              |              |                 |             |
              |              |                 |             |
              |(security exchange)             |             |
             1|<------------------------------>|             |
              |              |                 |             |
              |(a list of CEs and their attributes)          |
             2|<-------------------------------|             |
              |              |                 |             |
              |(a list of FEs and their attributes)          |
             3|------------------------------->|             |
              |              |                 |             |
              |              |                 |             |
              |<----------------Fl------------>|             |
          Figure 7. An example of message exchange over Fl reference point
        CE managers and FE managers may communicate across the Fl reference
        point in the pre-association phase in order to determine which CEs
        and FEs should communicate with each other.  Communication across
        the Fl reference point is optional in this architecture.  No
        requirements are placed on this reference point.
        CE managers and FE managers may be operated by different entities.
        The operator of the CE manager may not want to divulge, except to
        specified FE managers, any characteristics of the CEs it manages.
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        Similarly, the operator of the FE manager may not want to divulge FE
        characteristics, except to authorized entities.  As such, CE
        managers and FE managers may need to authenticate one another.
        Subsequent communication between CE managers and FE managers may
        require other security functions such as privacy, non-repudiation,
        freshness, and integrity.
        Once the necessary security functions have been performed, the CE
        and FE managers communicate to determine which CEs and FEs should
        communicate with each other.  At the very minimum, the CE and FE
        managers need to learn of the existence of available FEs and CEs
        respectively.  This discovery process may or may not entail one or
        both managers learning the capabilities of the discovered ForCES
        protocol elements. Figure 7 shows an example of possible message
        exchange between CE manager and FE manager over Fl reference point.
     5.1.2. Ff Reference Point
             FE Manager      FE               CE Manager     CE
              |              |                 |             |
              |              |                 |             |
              |(security exchange)             |(security exchange)
             1|<------------>|authentication  1|<----------->|authentication
              |              |                 |             |
              |(FE ID, attributes)             |(CE ID, attributes)
             2|<-------------|request         2|<------------|request
              |              |                 |             |
             3|------------->|response        3|------------>|response
              |(corresponding CE ID)           |(corresponding FE ID)
              |              |                 |             |
              |              |                 |             |
              |<-----Ff----->|                 |<-----Fc---->|
                          Figure 8. Examples of message exchange
                             over Ff and Fc reference points.
        The Ff reference point is used to inform forwarding elements of the
        association decisions made by FE managers in pre-association phase.
        Only authorized entities may instruct a FE with respect to which CE
        should control it.  Therefore, privacy, integrity, freshness, and
        authentication are necessary between FE manager and FEs when the FE
        manager is remote to the FE.  Once the appropriate security has been
        established, FE manager instructs FEs across this reference point to
        join a new NE or to disconnect from an existing NE. Figure 8 shows
        example of message exchange over Ff reference point.
        Note that when the FE manager function may be co-located with the FE
        (such as by manual keypad entry of the CE IP address), in which case
        this reference point is reduced to a built-in function.
     5.1.3. Fc Reference Point
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        The Fc reference point is used to inform control elements of the
        association decisions made by CE managers in pre-association phase.
        When the CE manager is remote, only authorized entities may instruct
        a CE to control certain FEs.  Privacy, integrity, freshness and
        authentication are also required across this reference point in such
        a configuration.  Once appropriate security has been established,
        the CE manager instructs CEs as to which FEs they should control and
        how they should control them. Figure 7 shows example of message
        exchange over Fc reference point.
        As with the FE manager and FEs, configurations are possible where
        the CE manager and CE are co-located and no protocol is used for
        this function.
     5.2. Post-association Phase and Fp reference point
        Post-association phase is the period of time during which a FE and
        CE have been configured with information necessary to contact each
        other and includes both communication establishment and steady-state
        communication. The communication between CE and FE is performed
        across the Fp ("p" meaning protocol) reference point. ForCES
        protocol is exclusively used for all communication across the Fp
        reference point.
     5.2.1. Proximity and Interconnect between CEs and FEs
        The ForCES Working Group has made a conscious decision that the
        first version of ForCES will not be designed to support
        configurations where the CE and FE are located arbitrarily in the
        network.  In particular, ForCES is intended for "very close" CE/FE
        localities in IP networks, as defined by ForCES Applicability
        Statement ([FORCES-APP]). Very Close localities consist of control
        and forwarding elements that either are components in the same
        physical box, or are separated at most by one local network hop.
        CEs and FEs can be connected by a variety of interconnect
        technologies, including Ethernet connections, backplanes, ATM (cell)
        fabrics, etc. ForCES should be able to support each of these
        interconnects (see [FORCES-REQ] Section 5, requirement #1). ForCES
        will make use of an existing RFC2914 compliant L4 protocol with
        adequate reliability, security and congestion control (e.g. TCP,
        SCTP) for transport purposes.
     5.2.2. Association Establishment
        As an example, figure 9 shows some of the message exchange that need
        to happen before the association between CE and FE is fully
        established. Typically, FE would need to inform the CE of its own
        capability and its topology in relation to other FEs. The capability
        of FE is represented by FE model, described in another separate
        document. The model would allow FE to describe what kind of packet
        processing functions it contains, in what order these processing
        happen, what kind of configurable parameters it allows, what
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        statistics it collects and what events it might throw, etc. Once
        such information is available to CE, CE sends all the necessary
        configuration to FE so that FE can start receiving and processing
        packets. For example, CE might need to send a snapshot of the
        current routing table to FE so that FE can start routing packets
        correctly. Once FE starts accepting packets for processing, we say
        the association of this FE with its CE is now established. From then
        on, CE and FE enter steady-state communication as described in
                     FE                      CE
                     |                       |
                     |(Hello, are you there?)|
                     |                       |
                     |(Yes. let me join the NE please.)
                     |                       |
                     |(Security exchange.)   |
                     |                       |
                     |(What kind of FE are you? -- capability query)
                     |                       |
                     |(Here is my FE functions/state: use model to describe)
                     |                       |
                     |(How are you connected with others? -- topology query)
                     |                       |
                     |(Here is the topology info)
                     |                       |
                     |(Config for FE initialization, e.g. routing table)
                     |                       |
                     |(I am ready to go. Shall I?)
                     |                       |
                     |(Go ahead!)            |
                     |                       |
               Figure 9. Example of message exchange between CE and FE
                         over Fp to establish NE association
     5.2.3. Steady-state Communication
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        Once an association is established between the CE and the FE, the
        ForCES protocol is used by the CE and the FE over Fp reference point
        to exchange information to facilitate packet processing.
                     FE                      CE
                     |                       |
                     |(Add these new routes.)|
                     |                       |
                     |(Successful.)          |
                     |                       |
                     |                       |
                     |                       |
                     |(Query some stats.)    |
                     |                       |
                     |(Reply with stats collected.)
                     |                       |
                     |                       |
                     |                       |
                     |(My port is down, with port #.)
                     |                       |
                     |(Route to this port instead...)
                     |                       |
                     |                       |
              Figure 10. Examples of message exchange between CE and FE
                      over Fp during steady-state communication
        Based on the information acquired through CEs' control processing,
        CEs will frequently need to manipulate the packet-forwarding
        behaviors of their FE(s) by sending instructions to FEs. For
        example, Figure 10 shows one such message exchange in which CE sends
        new routes to FE so that FE can add them to its routing table. CE
        can also query FE for statistics collected by FE and FE can also
        notify CE of some important events, like interface up and down, etc.
        Figure 8 also shows such examples.
     5.2.4. Data Packets across Fp reference point
        Control packets (such as RIP, OSPF messages) addressed to any of
        NE's interfaces are typically redirected by the receiving FE to its
        CE, and CE may originate packets and have its FE deliver them to
        other NEs. Therefore, the communication across the Fp reference
        point includes not only the control messages from CEs to FEs and the
        status or statistics report from FEs to CEs, but also the data
        packets that are redirected between them. Moreover, one FE may be
        controlled by multiple CEs. In this configuration, the control
        protocols supported by the FORCES NEs may spread across multiple
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        CEs. For example, one CE may support OSPF and another supports BGP.
        FEs are configured to recognize these protocol packets and forward
        them to the corresponding CE.
        Figure 11 shows one example of how the OSPF packets originated by
        router A are passed to router B. In this example, ForCES protocol is
        used to transport the packets from CE to FE inside router A, and
        then from FE to CE inside router B. In light of the fact that ForCES
        protocol is responsible to transport both the control messages and
        the data packets between CE and FE over Fp reference point, it is
        possible to use either a single protocol or multiple protocols to
        achieve that.
             ---------------------           ----------------------
             |                   |           |                    |
             |    +--------+     |           |     +--------+     |
             |    |CE(OSPF)|     |           |     |CE(OSPF)|     |
             |    +--------+     |           |     +--------+     |
             |        |          |           |          ^         |
             |        |Fp        |           |          |Fp       |
             |        v          |           |          |         |
             |    +--------+     |           |     +--------+     |
             |    |  FE    |     |           |     |   FE   |     |
             |    +--------+     |           |     +--------+     |
             |        |          |           |          ^         |
             | Router |          |           | Router   |         |
             | A      |          |           | B        |         |
             ---------+-----------           -----------+----------
                      v                                 ^
                      |                                 |
                      |                                 |
            Figure 11. Example to show data packet flow between two NEs.
     5.2.5. Proxy FE
        In the case where a physical FE cannot implement (e.g., due to the
        lack of a general purpose CPU) the ForCES protocol directly, a proxy
        FE can be used in the middle of Fp reference point. This allows the
        CE communicate to the physical FE via the proxy by using ForCES,
        while the proxy manipulates the physical FE using some intermediary
        form of communication (e.g., a non-ForCES protocol or DMA). In such
        an implementation, the combination of the proxy and the physical FE
        becomes one logical FE entity.
     5.3. Association Re-establishment
        FEs and CEs may join and leave NEs dynamically (see [FORCES-REQ]
        Section 5, requirements #12 and #13). When a FE or CE leaves the NE,
        the association with the NE is broken. If the leaving party rejoins
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        a NE later, to re-establish the association, it may or may not need
        to re-enter the pre-association phase. Loss of association can also
        happen unexpectedly due to loss of connection between the CE and the
        FE. Therefore, the framework allows the bi-directional transition
        between these two phases, but the ForCES protocol is only applicable
        for the post-association phase. However, the protocol should provide
        mechanisms to support association re-establishment (see [FORCES-REQ]
        Section 5, requirement #7).
        Let's use the example in Figure 5 to see what happens when the
        association is broken and later re-established again. Section 4.2
        already explains what happens if CE1 fails and how CE2 can take
        over. Note that if no CE redundancy is provided, FEs need to be told
        at the association establishment time what to do in the case of CE
        failure. FEs may be told to stop packet processing all together if
        its CE fails. Or, FEs may be told to continue forwarding packets
        even in the face of CE failure. No matter what, it needs to be part
        of the configuration when the association is established.
        Let's now look at the case when FE1 leaves the NE temporarily,
        assuming CE1 is the working CE for the moment. FE1 may voluntarily
        decides to leave the association. Or, it is more likely that FE1
        stops functioning simply due to unexpected failure. In former case,
        CE1 receives a "leave-association request" from FE1. In the latter,
        CE1 detects the failure of FE1 by some other mean. In both cases,
        CE1 would keep a note of such event for FE1 while continue
        commanding FE2. When FE1 decides to rejoin again, or when it is back
        up again from the failure, FE1 would need to re-discover its master
        (CE). This can be achieved by several means. It may re-enter the
        pre-association phase and get that information from its FE manager.
        It may retrieve the previous CE information from its cache, if it
        decides that the information is still valid. Or, that information
        can be simply hard-coded or pre-configured into it. Once it
        discovers its CE, it starts message exchange with CE to re-establish
        the association just as outlined in Figure 9, with the possible
        exception that it might be able to bypass the transport of the
        complete initialization information. Suppose that FE1 still have its
        routing table and other state information from the last association,
        instead of sending all the information again from scratch, it can
        choose to use more efficient mechanism to re-sync up the state with
        its CE. For example, a checksum of the state might give a quick
        indication of whether or not the state is in-sync with its CE. By
        comparing its state with CE first, it sends information update only
        if it is needed.
     6. Applicability to RFC1812
        [FORCES-REQ] Section 5, requirement #9 dictates that "All proposed
        ForCES architecture MUST explain how that architecture may be
        applied to support all of a router's functions as defined in
        [RFC1812]." RFC1812 discusses many important requirements for IPv4
        routers from the link layer to the application layer. This section
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        addresses the relevant requirements for implementing IPv4 routers
        based on ForCES architecture and how ForCES satisfies these
     6.1. General Router Requirements
        Routers have at least two or more logical interfaces. When CEs and
        FEs are separated by ForCES within a single NE, some additional
        interfaces are needed for intra-NE communications. Figure 12 shows
        an example to illustrate that. This NE contains one CE and two FEs.
        Each FE has four interfaces; two of them are used for receiving and
        transmitting packets to the external world, while the other two are
        for intra-NE connections. CE has two logical interfaces #9 and #10,
        connected to interfaces #3 and #6 from FE1 and FE2, respectively.
        Interface #4 and #5 are connected for FE1-FE2 communication. So this
        router NE provides four external interfaces (#1, 2, 7 and 8).
                     |               router NE       |
                     |   -----------   -----------   |
                     |   |   FE1   |   |   FE2   |   |
                     |   -----------   -----------   |
                     |   1| 2| 3| 4|   5| 6| 7| 8|   |
                     |    |  |  |  |    |  |  |  |   |
                     |    |  |  |  +----+  |  |  |   |
                     |    |  |  |          |  |  |   |
                     |    |  | 9|        10|  |  |   |
                     |    |  | -------------- |  |   |
                     |    |  | |    CE      | |  |   |
                     |    |  | -------------- |  |   |
                     |    |  |                |  |   |
                          |  |                |  |
                          |  |                |  |
                     Figure 12. A router NE example with four interfaces.
        IPv4 routers must implement IP to support its packet forwarding
        function, which is driven by its FIB (Forwarding Information Base).
        This Internet layer forwarding (see [RFC1812] Section 5)
        functionality naturally belongs to FEs in the ForCES architecture.
        A router may implement transport layer protocols (like TCP and UDP)
        that are required to support application layer protocols (see
        [RFC1812] Section 6). One important class of application protocols
        is routing protocols (see [RFC1812] Section 7). In ForCES
        architecture, routing protocols are naturally implemented by CEs.
        Routing protocols require routers communicate with each other. This
        communication between CEs in different routers is supported in
        ForCES by the FEs' ability to redirect data packets
        addressed to routers (i.e., NEs) and CEs' ability to originate
        packets and have them delivered by their FEs. This communication
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        occurs across Fp reference point inside each router and between
        neighboring routers' interfaces, as illustrated in Figure 11.
     6.2.Link Layer
        Since FEs own all the external interfaces for the router, FEs need
        to conform to the link layer requirements in RFC1812. Theoretically,
        ARP support may be implemented in either CEs or FEs. As we will see
        later, a number of behaviors that RFC1812 mandates fall into this
        category -- they may be performed by the FE and may be performed by
        the CE. A general guideline is needed to ensure interoperability
        between separated control and forwarding planes. The guideline we
        offer here is that CEs are required to be capable of these
        operations while FEs may or may not choose to implement them. FE
        model should indicate its capabilities in this regard.
        Interface parameters, including MTU, IP address, etc., must be
        configurable by CEs via ForCES. CEs must be able to determine
        whether a physical interface in an FE is available to send packets
        or not. FEs must also inform CEs the status change of the interfaces
        (like link up/down) via ForCES.
     6.3.Internet Layer Protocols
        Both FEs and CEs must implement IP protocol and all mandatory
        extensions as RFC1812 specified. CEs should implement IP options
        like source route and record route while FEs may choose to implement
        those as well. Timestamp option should be implemented by FEs to
        insert the timestamp most accurately. FE must interpret the IP
        options that it understands and preserve the rest unchanged for use
        by CEs. Both FEs and CEs might choose to silently discard packets
        without sending ICMP errors, but such events should be logged and
        counted. FEs can report statistics for such events to CEs via
        When multiple FEs are involved to process packets, the appearance of
        single NE must be strictly maintained. For example, Time-To-Live
        (TTL) must be decremented only once within a single NE. For example,
        it can be always decremented by the last FE with egress function.
        FEs must receive and process normally any packets with a broadcast
        destination address or a multicast destination address that the
        router has asked to receive. When IP multicast is supported in
        routers, IGMP is implemented in CEs. CEs are also required of ICMP
        support, while it is optional for FEs to support ICMP. Such an
        option can be communicated to CEs as part of the FE model.
        Therefore, FEs can always rely upon CEs to send out ICMP error
        messages, but FEs also have the option to generate ICMP error
        messages themselves.
     6.4.Internet Layer Forwarding
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        IP forwarding is implemented by FEs. After routing protocol update
        its routing tables at CEs, ForCES is used to send the new routing
        table entries from CEs to FEs. Each FE has its own routing table and
        uses this table to direct packets to the next hop interface.
        Upon receiving IP packets, FE verifies the IP header and process
        most of the IP options. Some options can't be processed until the
        routing decision has been made. Routing decision is made after
        examining the destination IP address. If the destination address
        belongs to the router itself, the packets are forwarded to CE.
        Otherwise, FE determines the next hop IP address by looking up in
        its routing table. FE also determines the network interface it uses
        to send the packets. Sometimes FE may need to forward the packets to
        another FE before packets can be forwarded out to the next hop.
        Right before packets are forwarded out to the next hop, FE
        decrements TTL by 1 and processes any IP options that cannot be
        processed before. FE performs any IP fragmentation if necessary,
        determines link layer address (e.g., by ARP), and encapsulates the
        IP datagram (or each of the fragments thereof) in an appropriate
        link layer frame and queues it for output on the interface selected.
        Other options mentioned in RFC1812 for IP forwarding may also be
        implemented at FEs, for example, packet filtering.
        FEs typically forward packets destined locally to CEs. FEs may also
        forward exceptional packets (packets that FEs don't know how to
        handle) to CEs. CEs are required to handle packets forwarded by FEs
        for whatever different reasons. It might be necessary for ForCES to
        attach some meta-data with the packets to indicate the reasons of
        forwarding from FEs to CEs. Upon receiving packets with meta-data
        from FEs, CEs can decide to either process the packets themselves,
        or pass the packets to the upper layer protocols including routing
        and management protocols. If CEs are to process the packets by
        themselves, CEs may choose to discard the packets, or modify and re-
        send the packets. CEs may also originate new packets and deliver
        them to FEs for further forwarding.
        Any state change during router operation must also be handled
        correctly according to RFC1812. For example, when an FE ceases
        forwarding, the entire NE may continue forwarding packets, but it
        needs to stop advertising routes that are affected by the failed FE.
     6.5.Transport Layer
        Transport layer is typically implemented at CEs to support higher
        layer application protocols like routing protocols. In practice,
        this means that most CEs implement both the Transmission Control
        Protocol (TCP) and the User Datagram Protocol (UDP).
        Both CEs and FEs need to implement ForCES protocol. If some layer-4
        transport is used to support ForCES, then both CEs and FEs need to
        implement the L4 transport and ForCES protocols. It is possible that
        all FEs inside an NE implements only one such protocol entity.
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     6.6. Application Layer -- Routing Protocols
        Both interior routing protocols and exterior routing protocols are
        implemented on CEs. The routing packets originated by CEs are
        forwarded to FEs for delivery. The results of such protocols (like
        routing table update) are communicated to FEs via ForCES.
     6.7. Application Layer -- Network Management Protocol
        RFC1812 also dictates "Routers MUST be manageable by SNMP." (see
        [RFC1157] Section 8) In general, for post-association phase, most
        external management tasks (including SNMP) SHOULD be done through
        interaction with the CE in order to support the appearance of a
        single functional device. Therefore, it is recommended that SNMP
        management agent be implemented by CEs and the SNMP messages sent to
        FEs be redirected to their CEs. This also requires that ForCES In
        certain conditions (e.g. CE/FE disconnection), it may be useful to
        allow SNMP to be used to diagnose and repair problems. However, care
        should be taken when exercising such mechanisms and guidelines are
        provided in [FORCES-REQ], Section 5, requirement #4.
     7. Summary
        This document defines an architectural framework for ForCES. It
        identifies the relevant components for an ForCES network element,
        including (one or more) FEs, (one or more) CEs, FE manager
        (optional), and CE manager (optional). It also identifies the
        interaction among these components and discusses all the major
        reference points. It is important to point out that, among all the
        reference points, only the interface between CEs and FEs is within
        the scope of ForCES. ForCES alone may not be enough to support all
        different NE configurations. However, we believe ForCES is the most
        important element in realizing the physical separation and
        interoperability of CEs and FEs, and hence the first interface that
        ought to be standardized. Simple and useful configurations can still
        be implemented with only CE-FE interface being standardized, e.g.,
        single CE with full-meshed FEs and static configuration without the
        need for CE/FE managers.
     8. Security Considerations
        The security necessary across each reference point except Fp is
        discussed throughout the document. In general, the physical
        separation of two entities usually requires much stricter security
        measurement in place. For example, we pointed out in Section 5.1
        that authentication becomes necessary between CE manager and FE
        manager, between CE and CE manager, between FE and FE manager in
        some configuration.  The physical separation of CE and FE also
        imposes serious security requirement for ForCES protocol. The
        security requirements for reference point Fp (i.e., ForCES protocol)
        are discussed in detail in [FORCES-REQ] Section 8.
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     9. Intellectual Property Right
        The authors are not aware of any intellectual property right issues
        pertaining to this document.
     10. Normative References
        [RFC2914] S. Floyd, "Congestion Control Principles", RFC2914,
        September 2000.
        [RFC1157] J. Case, et. al., "A Simple Network Management Protocol
        (SNMP)", RFC1157, May 1990.
        [RFC3084] K. Chan, et. al., "COPS Usage for Policy Provisioning
        (COPS-PR)", RFC3084, March 2001.
        [RFC1812]  F. Baker, "Requirements for IP Version 4 Routers",
        RFC1812, June 1995.
     11. Informative References
        [FORCES-REQ] T. Anderson, et. al., "Requirements for Separation of
        IP Control and Forwarding", work in progress, February 2002, <draft-
        [FORCES-APP] A. Crouch, et. al., "ForCES Applicability Statement",
        work in progress, February 2002, <draft-ietf-forces-applicability-
     12. Acknowledgments
        Joel M. Halpern gave us many insightful comments and suggestions and
        pointed out several major issues. Many of our colleagues and people
        in the ForCES mailing list also provided valuable feedback.
     13. Authors' Addresses
        Lily L. Yang
        Intel Labs
        2111 NE 25th Avenue
        Hillsboro, OR 97124 USA
        Phone: +1 503 264 8813
        Ram Dantu
        Netrake Corporation
        3000 Technology Drive
        Plano, Texas 75074
        Phone: +1 214 291 1111
        Todd A. Anderson
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        Intel Labs
        2111 NE 25th Avenue
        Hillsboro, OR 97124 USA
        Phone: +1 503 712 1760
        1. Abstract........................................................1
        2. Definitions.....................................................1
        3. Introduction to Forwarding and Control Element Separation
        4. Architecture....................................................6
           4.1. Control Elements and Fr Reference Point....................7
           4.2. Forwarding Elements and Fi reference point.................8
           4.3. CE Managers...............................................10
           4.4. FE Managers...............................................11
        5. Operational Phases.............................................11
           5.1. Pre-association Phase.....................................11
              5.1.1. Fl Reference Point...................................11
              5.1.2. Ff Reference Point...................................12
              5.1.3. Fc Reference Point...................................12
           5.2. Post-association Phase and Fp reference point.............13
              5.2.1. Proximity and Interconnect between CEs and FEs.......13
              5.2.2. Association Establishment............................13
              5.2.3. Steady-state Communication...........................14
              5.2.4. Data Packets across Fp reference point...............15
              5.2.5. Proxy FE.............................................16
           5.3. Association Re-establishment..............................16
        6. Applicability to RFC1812.......................................17
           6.1. General Router Requirements...............................18
           6.2. Link Layer................................................19
           6.3. Internet Layer Protocols..................................19
           6.4. Internet Layer Forwarding.................................19
           6.5. Transport Layer...........................................20
           6.6. Application Layer -- Routing Protocols....................21
           6.7. Application Layer -- Network Management Protocol..........21
        7. Summary........................................................21
        8. Security Considerations........................................21
        9. Intellectual Property Right....................................22
        10. Normative References..........................................22
        11. Informative References........................................22
        12. Acknowledgments...............................................22
        13. Authors' Addresses............................................22
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